This invention relates to semiconductor devices and, more particularly, to semiconductor devices including an integrated transistor, diode, and resistor.
In many applications it is usual to provide a diode across the collector and emitter of a transistor. For example, such a diode can provide over voltage protection for the transistor or can be used to carry the negative portion of load current in an inverter type application. With an NPN transistor, the rectifier anode is connected to the transistor emitter and the cathode is connected to the transistor collector; with a PNP transistor, the rectifier anode is connected to the transistor collector and the rectifier cathode is connected to the transistor emitter.
With high voltage switching transistors where fast turn off time is important, prior art transistors with integral diodes have not been entirely satisfactory. In one such device, one of the rectifier's electrodes is located in the center of the device and is integral with material forming the transistor base. This electrode is surrounded by a band of material integral with and forming part of the transistor emitter and the emitter contact extends across the band to the rectifier electrode. With this arrangement, a direct ohmic path is provided from the emitter contact to the base contact adjacent the band, that is, in the material forming the base and the electrode. The resistance of the path insures that the emitter is not inadvertently turned on by low base signals.
Because the band is part of the emitter it injects current when the transistor is on. When the transistor is turned off, the stored charge under the emitter flows laterally to the base electrode as part of the reverse base current flow. The width of the band adds to the lateral distance that must be traversed by the stored charge which in turn adds to the turn off time. Attempts to reduce the width of the band and, thus, the turn off time of the transistor have resulted in a relatively low resistance of the ohmic path between the emitter and base contacts whereby an excessively high base signal is required to turn on the transistor.